This thesis focuses on enhancing the predictive accuracy of the ShorelineS one-dimensional (1D) coastline model, which is used to simulate coastal evolution. The research primarily addresses the limitation of ShorelineS, which excludes cross-shore sediment redistribution, impacting its ability to predict coastal changes, as demonstrated in the case of the Bacton Sandscaping project.

The Bacton Sandscaping project was designed to counter coastal erosion threatening the Bacton Gas Terminal, a crucial part of the UK's energy infrastructure. Instead of traditional hard engineering solutions, a soft approach, inspired by the Dutch Sand Engine, was chosen. This method involves depositing a large volume of sand along the coast, which is gradually redistributed by natural forces to offer long-term protection. Despite the project’s success, challenges were identified in modeling the coastline's evolution, particularly with ShorelineS, which only considers longshore sediment transport. This thesis explores how incorporating cross-shore redistribution into ShorelineS can improve its predictive capability, especially for large-scale nourishment projects like Bacton.

The research begins by evaluating ShorelineS in its current form, comparing modeled and observed coastline changes. The model tends to underestimate erosion near the gas terminal and overestimate sediment distribution to nearby villages, leading to inaccuracies. To address this, a redistribution factor (R-factor) is introduced, which adjusts the model based on observed cross-shore changes. The R-factor is applied to the terminal section, improving predictions slightly, though overall performance remains limited due to the model's sediment conservation assumptions.

The study concludes that cross-shore redistribution significantly affects coastline evolution, as seen in the Bacton case. Although the R-factor improves predictions somewhat, the limitations of ShorelineS, such as its sediment retention assumptions and the CERC2 transport equation, suggest that further model refinements are necessary. Future research should explore alternative approaches to cross-shore redistribution, more frequent coastline updates, and sensitivity analyses to enhance the model's accuracy in predicting the evolution of mega nourishment projects.

Safety assessments are taken every five years to examine whether the coast is safe enough. After investigation, it was found that one of the weak spots is the Hondsbossche and Pettemer Sea defence (from hereafter referred to as HPZ) which required reinforcement at this location. Therefore, a mega nourishment was implemented in 2015 to ensure the safety of the hinterland. The implementation of this mega nourishment created the Hondsbossche Dunes (from hereafter referred to as HD). After the implementation of the mega nourishment, it was soon found that the width of the beach decreased. The beach width is defined as the horizontal distance between the waterline and the dune foot position which is defined at elevation NAP + 3.0 m. The decrease in beach width occurred mainly on the edges where there is a stronger curvature, so more change in shoreline orientation, compared to the other parts of the nourishment. As a result of the decrease in beach width, an additional nourishment was implemented in 2018 to meet the requirements. These are not the safety requirements but the beach width is important for recreation so the beach cannot be too narrow.

The aim of this research is to improve understanding of beach width reducing processes at curved coastlines to improve predictions of beach width on a scale of ~5 years after implementing a nourishment at the HD. The relationship between the curved coastline and long waves and its influence on the change in beach width was investigated. Special attention is given to the role of infragravity waves. Hereto the numerical model XBeach is used which can simulate morphodynamics with and without infragravity forcing. From the literature review, it follows that long waves, also called infragravity waves, are formed from small waves. A difference between infragravity waves and short waves is that the processes of the infragravity waves take place mainly in the surf zone and swash zone, while most of the short waves are dissipated in the surf zone and swash zone.

To determine the influence of coastline curvature and infragravity waves on beach width change, two types of models were run. The first model was a schematic model where an alongshore uniform coast is applied in the model to neglect the influence of alongshore variability in cross-shore profiles. In addition, multiple runs were performed varying the strength of the curvature and the mode (stationary mode, so without infragravity waves and groupiness, or surfbeat with infragravity waves) so that the results could be compared. This study showed that including infragravity waves results in a different predicted beach width change. In addition, stronger curvature leads to larger gradients in longshore sediment transport and thus the change in beach width.

The second model run was the HD’s complex model where the conditions as present in reality were applied, i.e., bathymetry and wave conditions. Based on the XBeach model of the HD, it can be concluded that the prediction is reasonably similar for the first year of the simulation. The first year of the simulation is the second year after the nourishment is implemented, i.e. 2016, so initial effects are not included in the result of the measurements. However, the predictions and measurements hardly match in the other years resulting in a very low correlation which follows from the validation.

Galveston Island is crucial for sea turtle nesting but faces erosion and increasingwater levels, limiting nesting opportunities and posing dangers to nests in flood-prone areas. A numerical model was developed to estimate groundwater levels and hydrodynamics at specific locations on two onedimensional transects, located at the Tipsy Turtle and the Sea Wall. The model was used to assess the risk of turtle nest inundation at six fictitious nest locations and compared to a previous method used in a study by Ware et al. (2019), which used the 𝑅2% formulation developed by Stockdon et al. (2006).
The model was validated using data from a two-month field campaign in Galveston, Texas. Two five-day periods were simulated to represent different tidal and wind conditions. The model was also simulated for five different hydraulic conductivities.
In the nearshore, hydraulic conductivity did not influence the results. Although mean surface
elevations were captured well, the infragravity and incident wave heights were exaggerated in most cases, possibly due to wave spreading not being accounted for in 1D models and, therefore, concentrating energy transfer of incident waves to the infragravity waves.
The model demonstrated that small hydraulic conductivities performed better in estimating
groundwater levels in calmer conditions when the mean sea level was considerably lower than
the groundwater level, as in the second period. The RMSE close to the dune foot was 0.075 m for 𝐾 = 0.0001 m/s and 0.4 m for 𝐾 = 0.001 m/s in period 2 close to the dune foot at the Sea Wall. In contrast, the intermediate and higher hydraulic conductivities showed minor errors in more dynamic conditions close to the shoreline in period 1. The RMSE at TT4 (approximately halfway at the Tipsy Turtle) during period 1 was 0.164 m for 𝐾 = 0.001 m/s and 0.239 m for 𝐾 = 0.0001 m/s. In all cases, the best hydraulic conductivity decreased in landward direction.
Not a single hydraulic conductivity performed well in all conditions. Therefore, the results in a
test case assessing turtle nest inundation also varied. In most cases, the model overestimated turtle nest inundation, while the method by Ware et al. (2019) underestimated in all but one case. The run-up estimations of Stockdon et al. (2006) as used by Ware et al. (2019) were sensitive to changes in bed slope, with a steeper slope of 0.05 aligning better with modelled run-up during high tide in period 1 and a flatter slope of 0.01 in period 2. A slope of 0.0316 aligned mainly with the modelled run-up in intermediate conditions in period 1.
This report demonstrated that not one of the cases simulated is reliable enough for assessing turtle nest flooding accurately in all hydrodynamic conditions and is too conservative in most, but not all, cases. Therefore, the current model cannot be used as an alternative yet. Moreover, it concluded that the method using the empirical run-up formulation as in Ware et al. (2019) should also be used cautiously due to consistent underestimations in this experiment.

Low-lying reef-lined coasts are vulnerable to coastal flooding. While fringing reefs usually protect coastal communities from moderate storms by dissipating incoming wave energy, they can exacerbate flooding during strong events with energetic waves. Low-frequency waves (0.001-0.04 Hz) play an important role in coastal hazards, dominating nearshore reef flat wave patterns. Furthermore, low-frequency resonance on reef flats has been suggested to exacerbate coastal hazards. Despite expanding knowledge of wave dynamics on coral reefs, the transient characteristics of low-frequency resonant oscillations on reef flats have not been extensively researched, and no methods currently exist for analyzing this phenomenon in field observations.

This thesis introduces a new methodology for assessing transient characteristics of resonant and energetic low-frequency oscillations using the Hilbert-Huang transform. The study is based on a 5-month dataset from a cross-shore transect of a reef at Roi-Namur in the Marshall Islands, where the reef geomorphology corresponds to a natural frequency in the very low-frequency range (VLF, 0.001-0.005 Hz).

The findings indicate that resonance in the dataset typically had a short persistence, with a median and 90th duration of approximately 5 and 10 minutes respectively. Normalised with the period of the oscillations, which have a typical frequency of 0.0035 Hz, these durations are equal to approximately 1 and 2 wave oscillations. While these results are consistent with previous notions about the potentially short durations of resonance for this particular reef and dataset, they contradict suggestions that resonance is associated with a build-up over several wave periods. Comparison of resonance durations with instances of coastal flooding at the site shows that resonance occurred for about 1 hour within a 24-hour recording period during overwash and large runup events, but also at other times. The duration of energetic VLF oscillations, in general, was more closely linked to the severity of coastal flooding events. While resonance did not directly correspond to coastal flooding, it was observed that energetic VLF oscillations persisted for longer durations under (close to) resonant conditions.

These results suggest the critical role of energetic VLF waves in coastal hazards, while resonance may be of secondary importance. However, it is important to acknowledge the limitations of this study, which focused on a single dataset from a specific reef geometry and included short-duration recordings (34 minutes). Moreover, this study demonstrates the application of the Hilbert-Huang transform for analysing sea surface elevation signals recorded at a coral reef.

The insights gained from this research underscore the importance of considering both energetic VLF oscillations and resonance in coastal hazard assessment. Future investigations should explore factors influencing the persistence of such oscillations, like the characteristics of incoming wave forcing, to enhance coastal hazard prediction. Furthermore, the Hilbert-Huang transform offers a versatile framework for exploring more aspects regarding the time-varying and non-linear behaviour of waves on coral reefs.

The methodology framework proposed in this study facilitates the bulk analysis of extensive field data and longer sea surface elevation recordings, enabling further exploration of reef morphology and hydrodynamic controls on the persistence of energetic and resonant VLF oscillations.

In 2022, the Eastern Scheldt barrier was assessed regarding its compliance with the requirements in the Environment and Planning Act. The result of this assessment, mentioned in HKV et al. (2022), was that the barrier barely met the standards in failure probability: a value of 1/13.057 years while the minimum requirement is 1/10.000 years and the signal value is 1/30.000 years. This leaves a margin of a factor 1.3 increase for the current failure probability. The upper trusses of gates Scharen 13 (S13) untill Scharen 16 (S16) proved to be the weakest link in the barrier and contributed most to the total failure probability. This study only focuses on the upper truss of gate S13, and the results are assumed to be a rough indication of how the barrier responds as a whole.

This research investigates the usage of alternative wave load related model choices and their effect on the structural failure probability of the upper truss of gate S13. Combining these two aspects provides an estimate of the uncertainty in the estimated 1/13.057 years. Four areas within the current wave load related framework were considered.

- Dynamic wave pressure model. This aspect considers how the waves translate to dynamic pressures on the gates. Sainflou, quasi-regular linear wave theory (LWT), New Wave LWT η_recalculated and a spectral LWT are considered suitable alternatives compared to SainflouOSK, which is the currently used method. The linear models, especially the spectral variants, have the most complete representation of the physical processes. The spectral methods need the complete wave spectra as input, which is not yet accurately reproduced by the wave propagation model. New Wave LWT η_recalculated is particularly recommended because it provides a spectral calculation that accounts for the effects of steep waves. Sainflou corresponds best with a small, but high quality dataset of average pressures during regular waves. The linear theories perform best with a larger, but lower quality dataset of pressures during six hours in one storm. However, as no wave conditions were measured during this specific storm, wave propagation model output was used to employ this dataset. It is advised to solve the inconsistency between the data sources by performing scale model tests or SWASH simulations. Especially the performance under steep regular waves and bimodal spectra is crucial, as this study shows that these are the conditions that prevail during relevant storms. The structural failure probability changes significantly when these other dynamic wave pressure models are employed. Sainflou results in a factor 10 increase in failure probability for the upper truss of gate S13 and spectral LWT in a factor 100 decrease.

- Wave height distribution. This distribution is applied to move from a significant wave height to a maximum wave height within a storm. Two alternative distributions were considered, the Rayleigh distribution and the Battjes-Groenendijk distribution. The Rayleigh distribution, which is currently used in the assessment, is demonstrated to be conservative compared to six hours of wave pressure measurements in one storm. The Battjes-Groenendijk distribution could be an alternative, even though not validated in this study, leading to a factor five decrease in failure probability for the upper truss of S13. Given the significant impact on the failure probability and the indication of room for improvement, it is advised to perform further research in which wave height distribution is most suitable. SWASH simulations could be used to validate an alternative distribution. 

- Wave propagation model. Wave propagation is computed with SWAN considering wind-induced wave generation, whitecapping, depth-induced breaking, bottom friction, triad wave-wave interactions and quadruplet wave-wave interactions. From these source terms, adjustments to the formulations of wind-induced wave generation, whitecapping, depth-induced breaking and triad wave-wave interactions are considered. Modifying the CDS2 KOMEN whitecapping parameter within the current SWAN settings, which increases the whitecapping dissipation, improved the bias correction for both the wave height and period. However, this SWAN setting is not considered a better alternative. This is due to the lack of physical substantiation and its inability to reproduce measured spectral characteristics, especially during high water levels in storm conditions. The DCTA triad wave-wave interaction formulation showed most promising results in reproducing the bimodal spectra measured during severe storms. Additional research to improve the calibration of the DCTA formulation is recommended over employment of the adjusted CDS2 parameter. Using four other variations in SWAN settings with their corresponding bias corrections resulted in a spread of failure probabilities for the upper truss of S13 ranging from a factor 1.2 decrease to a factor five increase. 

- Wave load-related uncertainties and biases. This concept relates to the methodology of how uncertainties in parameters (probabilistic or deterministic) and models (bias corrections) are taken into account. Using energy interpolation over wave height interpolation between SWAN outcomes is considered an improvement for estimating the bias corrections. It leads to a factor 1.1 reduction in failure probability for the upper truss of gate S13. Similarly, bias estimation of the wave height and period by using measurements with an additional restriction on the water levels resulted in a factor 1.1 decrease. However, incorporating the entire distribution of wave heights, not only the mode of the chosen distribution, is considered an advancement that increases the failure probability by a factor two.

When combining the evaluations of alternative wave load related models and their effect on the structural failure probability of the upper truss of gate S13, it can be concluded that there is a large scatter around the 1/13.057 years. This uncertainty is a consequence of the inability to draw definite conclusions on which SWAN setting, dynamic wave pressure theory and wave height distribution to choose. There are well evaluated choices that decrease the failure probability below the signal value, but also other interesting options that increase it beyond the minimal requirement. Further research and data collection is necessary to determine which of these two scenarios is most likely. This study illustrates the starting point and direction for these next steps, while also providing the motivation and urgency to take them. 

Mangrove forests are highly valuable environments that offer numerous ecosystem services, including carbon storage, serving as nurseries for fish and shrimp, and providing timber. Additionally, these ecosystems offer coastline protection against erosion and flooding by attenuating waves and trapping sediment. Despite the crucial role of mangrove ecosystems, global mangrove cover has significantly declined in recent decades, with deforestation and coastal erosion being one of the contributing factors, launching a variety of initiatives for mangrove rehabilitation. To rehabilitate mangroves along eroding coastlines, permeable structures are designed as a solution. These structures dissipate incoming waves and reduce current strength, creating low-energy areas behind them that are conducive to sediment deposition. However, these structures frequently prove ineffective in the rehabilitation of mangroves, possibly due to the absence of clear design guidelines, and a lack of understanding regarding the influence of design parameters on the performance of these structures.

This study, therefore, aimed to develop an integrated design guideline for permeable structures aimed at rehabilitating mangrove habitat along eroding coastlines by combining scientific knowledge with insights gleaned from pilot projects conducted globally. Furthermore, the research aimed to deepen understanding of the influence of design parameters on the performance of these permeable structures by conducting simulations of various structure configurations in a numerical, process-based model, and evaluating the performance of each configuration in reducing bed shear stress across the sheltered areas behind them. A nested model was employed in Delft3D-4, configuring the flow and wave modules to be coupled in order to incorporate the interaction between waves and currents.

The developed conceptual framework and design guideline as well as the identified correlations between design parameters and structure performance in this study contribute to enhancing the design process of permeable structures aimed at rehabilitating mangrove habitat. It provides preliminary insights, highlights significant knowledge gaps and provides recommendations for further research, thereby opening up opportunities to expand knowledge surrounding the design of permeable structures aimed at morphodynamic restoration of mangrove habitat.

In this thesis, the identification and generation of meteotsunamis on the Southern North Sea are investigated. Although meteotsunamis are well known in other areas, hardly any research has been done on them near the Dutch Coast. Here, nine years of sea level elevation data are evaluated using the classic approach and a subsequent wavelet analysis to identify a list of potential meteotsunamis. Six of those events have been selected, for which the atmospheric conditions were simulated by WRF. To definitively classify them as meteotsunamis, a pressure jump and atmospheric front responsible for the generation are identified. The six events are characterized by computation of their angle of incidence, wave period, and wave height, and are checked for the probability of Proudman resonance. The six selected events were all linked to a pressure jump and an atmospheric front, and thus classified as meteotsunamis. Two events propagated from the North-West and the other four from the South through the English Channel. The wave periods varied between 10 and 25 minutes, and the wave heights varied around 0.20 meters, with a maximum wave height of 0.63 meters. Three events were linked to a front propagating overseas, all of which had a high probability that Proudman resonance contributed to their amplification. These results form a foundation for future research on meteotsunamis on the Southern North Sea.

Storm surges cause elevated water levels, potentially leading to dune erosion, which is crucial to understand due to the protective function of dunes and sandy coastal defences against flooding. Dunes are dynamic systems, extensively studied through flumes and field observations. Collecting hydrodynamic and morphological data from erosion-prone land reclamations can inform dune research and design optimisations. Loss of volume in land reclamation projects can result in setbacks, increased costs, and emissions. While dune erosion models estimate volume changes, they may deviate significantly under annual storm conditions due to calibration with normative storm conditions. Dune erosion typically occurs when the initial dune foot elevation is exceeded, resulting in a post-storm dune foot elevation at the maximum water level within the storm. Therefore, the (initial) dune foot might play an important role in the resulting erosion volume. The current knowledge on the dune foot behaviour of sandy coastal defences due to annual storm conditions is too limited and therefore the main research question of this thesis is: ‘How does the dune foot of sandy coastal defences behave due to annual storm conditions and what is the influence on dune erosion?’

This thesis performs an analysis of two field sites with hydrodynamic and morphological measurements, RealDune-REFLEX on the Holland coast and Land Reclamation Philippines in Manila Bay in the Philippines. For Land Reclamation Philippines, the hydrodynamics are transformed using two hydrodynamic instruments and validated using an ERA5 wind-driven SWAN model. The definition of the dune foot position is based on the maxima of curvature of the dune profile above a certain threshold following the second derivative method. The maximum total water level elevation is approximated with the measured water level elevation and an empirical parametrisation of the wave runup based on offshore wave conditions and the foreshore slope.

For RealDune-REFLEX, negligible erosion occurred for maximum total water level elevations far below the dune foot. For elevations just below and exceeding the dune foot, it shows that the post-storm dune foot correlates with the maximum total water level elevations. The dune foot could translate upwards by dune erosion and downward by bed level lowering and occasional avalanching. At Land Reclamation Philippines, it was found that pre-storm, more alongshore variability was observed in the vertical and horizontal position of the dune foot compared to post-storm. Minor coastal features eroded and resulted in larger dune foot retreats on those transects. Major coastal features remained present and influenced the post-storm dune foot position.

Concluding, an increase in the maximum total water level elevations above the initial dune foot height led to the highest erosion quantities of the dune front. However, no significant linear relation was found between the initial dune foot position relative to the maximum water level elevation and the resulting dune erosion. The post-storm dune foot elevation was found to correlate significantly to the maximum total water level elevation, approximated with an empirical runup formula, for upward and downward dune foot translation due to an out-of-equilibrium upper foreshore slope. Change of the horizontal dune foot position was found to relate to some extent to the storm duration and intensity of the total water level elevation exceeding the dune foot. Dune foot retreat magnitudes can be of the same order for small and large values of this quantification method and therefore exceedance of the dune foot does not provide a proper estimation of the dune erosion volume. It is found that the mismatched volume of the initial profile with the equilibrium condition, defined as an equilibrium slope reaching from the intersection of the initial profile with the maximum total water level elevation, is a more accurate estimation of the dune erosion volume. This methodology provides a practical way of predicting erosion volumes in sandy coastal defences using simple initial conditions, making it applicable to engineering practice.

For further research, it is recommended to improve the mismatch method based on the shape and size of the equilibrium profile. Second, study the dune foot dynamics within a storm using continuous LiDAR laser measurement and therewith validate dune erosion models. Third, civil contractors should enhance the quality of surveys in periods where dune erosion is expected to establish valuable datasets. Lastly, numerical modelling of complex typhoon-induced nearshore hydrodynamics could lead to more accurate insights into the alongshore behaviour of typhoon-induced dune erosion and the validity of the methods used.

Along the Dutch coastline, coastal dunes act as the primary sea defence to protect the low-lying areas behind it from flooding. To ensure that these dunes are strong enough to meet national safety standards, models are used to assess their strength. Even though our modelling capabilities have significantly improved over the years, model uncertainties remain because specific processes are not yet fully understood.

One of those uncertainties is the effect of sediment grain size on dune erosion. The relative importance and effect of this parameter differs significantly among dune erosion models. It is however expected that this parameter influences the morphological response of dunes during storm conditions.

Manipulative field experiments to study the effect of sediment grain size on dune erosion were conducted from September 24 until September 27 2022. In total, six experiments were conducted in which four different grain sizes were tested. Two open shipping containers were placed in the intertidal zone and acted as coastal wave flumes. Inside both containers, artificial dunes were built, with median grain sizes ranging from 0.21 to 0.31 mm, and were subjected to wave attack during high water. Cameras were used to calculate erosion volumes at a high sampling frequency. Environmental conditions were measured to capture water levels and wave conditions.

Dune erosion was measured with cameras, providing a low-cost and user-friendly method. This method can accurately and continuously capture dune profiles, particularly when there is a distinct color contrast between the dune profile, incoming swash, and backdrop. Analysis of the profile development reveals that the dune toe linearly translates over time in both the cross-shore and vertical directions. The translation in the vertical direction is also correlated with the rising mean water level.

The analytical model derived by Larson et al. (2004) based on wave impact theory was used to relate measured erosion volumes to incoming hydrodynamic conditions and to isolate the relative importance of sediment grain size on dune erosion. The considered time interval used in this study differs from the original approach, as this study focuses on short-term timescales of subsequent events during the experiment rather than the total erosion of an experiment. The application of this newly introduced timescale requires less data and gives more insight into the involved processes for subsequent erosion events. A linear trend is obtained between erosion volume and hydrodynamic forcing but the model cannot be used as an accurate prediction model, it tends to overestimate erosion volumes. The effect of grain size is incorporated in the transport coefficient. The transport gradient decreases when the sediment grain size increases. A negative correlation that converges as grain size increases was found between the two
parameters, demonstrating a non-linear, monotonic dependence.

This study reveals that erosion rates are smaller for dunes composed of coarser grains than finer grains. This is indicated by both the cumulative erosion volumes for identical hydrodynamic conditions and the values of the transport coefficient in the analytical model. Additionally, the formation of steeper
slopes is observed for dunes composed of coarser grains. These findings could be attributed to the fact that coarser grains have larger fall velocities and therefore are less easy transported by the waves and currents. This study enhances our comprehension of the effect of sediment grain size, ranging from 0.21 to 0.31 mm, on dune erosion and can contribute in resolving discrepancies between model predictions and field measurements to improve future assessments of dune erosion with models.

Many coasts around the world experience coastal erosion as a result of climate change, lack of sediment supply or human interventions. In the coming decades, coasts will be exposed to increased natural forcing because of climate change and sea level rise, leading to increased risks of coastal flooding and erosion. As sandy coasts are highly dynamic systems, it is difficult to assess the impact of these risks. Long-term coastal evolution is evaluated on large temporal and spatial scales, which introduce uncertainty. Hence, probabilistic estimates are required rather than single, deterministic values. Complex, process-based models have been developed to simulate morphological changes in coastal zones, but these come at large computational expense. As probabilistic approaches require a large number of simulations, it is necessary to reduce computational times as much as possible.

Variability in coastline position is induced by both cross-shore and longshore processes, which occur perpendicular and parallel to the coastline, respectively. Extreme storm events and sea level rise are responsible for the long-term cross-shore coastal dynamics, while gradients in longshore sediment transport act as the main driver for the longshore coastline changes. This study focuses on developing a framework to combine predictions of long-term cross-shore and longshore processes in a probabilistic way. The framework combines probabilistic predictions of cross-shore effects with deterministic estimates of longshore-induced coastline change. Two separate models are studied for the cross-shore and longshore component, namely the Probabilistic Coastline Recession (PCR) model and the ShorelineS model. Both models are computationally efficient due to the use of reduced complexity sediment transport formulations and a simplified representation of the beach profile.

The developed framework is validated by a case study of the IJmuiden coastline, the Netherlands, to assess the effects of the blockage of longshore sediment transport by the breakwaters between 1967 and 2007. Consequently, projections of coastline development are determined for the period of 2023 to 2100, for a range of IPCC sea level rise scenarios. To model storm erosion with the PCR model, a synthetic time series of storm events is simulated by establishing joint probabilities of wave forcing variates. Moreover, the PCR model provides stochastic estimates of coastline recession, enabled by the large number of simulations. Regarding ShorelineS, a representative wave climate based on data of local wave conditions was used to compute the longshore sediment transport.

The ShorelineS simulation over the hindcasted period clearly resulted in accretion on both sides of the breakwaters in the order of 100 to 900 meters, in accordance with field observations. This outcome indicates that limited threats of coastal erosion are currently present. However, when analysing the projections for 2023-2100, the PCR model predicted that a combination of storm impact and sea level rise can definitely lead to coastline recession, with a median retreat of 18 meters and 32 meters for the RCP2.6 and RCP8.5 sea level rise scenario, respectively. Moreover, the PCR model indicated that at some point the coastline erosion might be larger than 100 meters with a probability of 1.5% for RCP2.6 and 4% for RCP8.5. The combined projections confirm the importance of accounting for both cross-shore and longshore impacts on long-term coastline evolution and show that the developed framework is a useful method for this purpose.

For further research it is recommended to study long-term coastline evolution while including interaction between cross-shore and longshore processes, to analyse its contribution to long-term coastline changes. It is also advised to further investigate how to obtain stochastic projections of coastline development induced by longshore processes, as this study only presents the cross-shore component in a probabilistic way. Furthermore, stochastic projections of sea level rise and beach recovery can be studied as well.

When two storms quickly succeed one another, it can be called a twin storm. Storm conditions at the coast can lead to higher water levels and waves that can give rise to dune erosion. Therefore, after a storm the dunes are smaller than before the storm. Dune accretion is a much slower process than dune erosion; it can take weeks to months of time for a dune to grow the volume of sand it lost during a storm. If the interval time between two storms is smaller than the time it takes for a dune to gain the same volume of sand it loses during the storm, the second storm can lead to additional impact. So when storms quickly succeed one another, there is not enough time for the dune to significantly grow in between the storms. Due to additive impact, a twin storm might lead to more damage than a single storm with the same return period. In this research it is investigated whether twin storms give more dune erosion than a single storm with the same return period.

To answer this question, the research is divided into two parts. The first part consists of creating representative storm schematisations for single storms and for twin storms using simulated weather data from the European Centre for Medium Range Weather Forecasts (ECMWF). To start with, the storms from the dataset are divided into several clusters. For each cluster, one schematisation is made that represents the storm within that cluster.
In part two, the storm schematisations are used to calculate the dune erosion for each defined storm cluster. The dune erosion is modelled using XBeach, which is a model that is developed to simulate the evolution of the dune profile during extreme storms. The dune erosion will be modelled for two locations along the Dutch coast: Hoek van Holland and Nieuwvliet-Groede.

In this research, it turns out that at Hoek van Holland 25-30% of all storms can be considered a twin storm. At Nieuwvliet-Groede, 15-20% of all storms are twin storms. For Hoek van Holland very few of the constructed twin storms (approximately 10%) give more dune erosion than the constructed single storms. At location Nieuwvliet-Groede approximately 30% of the twin storms cause a larger dune erosion volume than the single storms. In most cases where twin storms result in more erosion than a single storm, this effect can be attributed to deviations in the storm schematisations.

For further research it’s recommended to investigate how the prediction of the time evolution of the water level can be improved. This is relevant because the dune erosion volume is strongly correlated with the water level. Furthermore, it is advised to do more research into the time scales and modelling of dune growth. Storm groups with interval periods of multiple weeks can be investigated with better predictions of dune growth.

The Wadden Sea serves multiple roles: it acts as a protective barrier against severe wave conditions, is a natural habitat for diverse flora and fauna and has recreational purposes. This research provides a comprehensive analysis of the intricate morphological dynamics of the Wadden Sea. By implementing various model schematizations, the study aims to enhance understanding of complex interactions between tidal inlets and basins, sediment transport dynamics, the influence of forcing actors and the importance of grid resolution. The ultimate goal is to improve the process-based models that are used to perform morphological forecasts, serving as a valuable tool for efficient coastal management and ensuring the preservation of this unique tidal ecosystem.

The study incorporates several schematizations, including the spatial extent of the modelling grid (domain schematization), grid resolution and various forcing components applied to the model boundaries and grid. Analysing water and sediment exchange between basins and through the Ameland tidal inlet has revealed distinct differences in hydrodynamics and morphodynamics across the model schematizations.

The single-inlet domain schematization encompassed only the Ameland tidal inlet region, while the more expansive multi-inlet schematization captured the entire Wadden Sea area. The single-inlet schematization does not account for hydrodynamic interactions between the Ameland basin and neighboring basins. This impacts sediment transport and morphological development in the Ameland inlet. A multi-inlet approach, encompassing the entire Wadden Sea, grants a more comprehensive model. The model with Wadden Sea domain schematization is essential for obtaining accurate results, as it showed significant differences in sediment transport compared to the single-inlet model. Modeling practices that exclude transport dynamics between tidal basins risk misrepresenting the natural processes in play.

Around the Ameland inlet, ebb dominance is observed in most regions, but there are exceptions in the marginal flood channel and at the outer side of the ebb-tidal delta. The Ameland model shows more ebb-dominance than the Wadden Sea model. Sediment transport during ebb periods exceed than during flood periods and differences in sediment transport between model schematizations are more pronounced during ebb than during flood.

Regarding the drivers of change, the spring-neap tidal cycle is particularly indicative of periods with meaningful residual sediment transport. Wind, especially from southwesterly directions, has a significant impact on water transport dynamics and a limited impact on sediment transport through the inlet.

Two grid schematizations are used, one with a base resolution of 60 by 60 meter and another with a high-resolution grid of 30 by 30 meter near the inlet. An enhanced grid resolution results in marginal changes in simulation outcomes, offering limited advantages for the conducted short-term morphological forecasts.

Overall, enhanced model schematizations can significantly improve short-term morphological forecasts of the Ameland tidal inlet, providing a more accurate and comprehensive representation of the complex hydrodynamic and sediment transport processes. Leveraging high-performance computing (HPC) for executing Delft3D-FM models has been central in this research, offering possibilities to enhance model resolution and reduce computational time.

The benefits of HPC come with the added challenge of mastering new software, handling larger models and interpreting more extensive data sets. A deep understanding of the physical processes and careful cost-benefit consideration is crucial, as increased computational power does not automatically resolve all modeling limitations. As such, the effective use of HPC in coastal modeling requires a balance between its sophisticated capabilities and the complexity it introduces.

Sea turtles, with six of the seven species listed as endangered by the IUCN, face significant threats. Their eggs exhibit sensitivity to fluctuations in temperature, salinity, and moisture during their 6-8-week incubation period. Inundation of these eggs can substantially decrease their viability and influence sex ratios. This thesis centers on mitigating the wave runup-driven flooding of sea turtle nesting beaches. Mitigating nest flooding can involve relocating at-risk nests to higher elevations. This thesis aims to assist in responsible nesting relocation by providing flood risk information as a function of beach elevation. The study site, Ras Baridi, is a significant nesting site for green turtles (\textit{Chelonia mydas}) in the Red Sea. The research focuses on the development of a methodology for assessing flood risk in environments with limited data.

The risk of sea turtle nests to flooding at Ras Baridi was assessed by employing two different metamodels, BEWARE 2.0 and HyCReWW, that were recently developed to assess wave runup in coral reef environments at low computational costs. These models employ different strategies to model wave runup. BEWARE 2.0 uses real bathymetry, whereas HyCReWW schematizes the input bathymetry. 40-year hindcast datasets of waves and water levels from 1978-2018 were used in combination with 10 m horizontal resolution bathymetry data from the Allen Coral Atlas as model inputs. HyCReWW exhibited maximum runup at an 80 m reef width (0.95 m for 1 year return period (RP), 1.95 m for 40 year RP) BEWARE 2.0’s highest runup was at an 80 m reef width (1.35 m for 1 year RP, 2.7 m for 40 year RP). Median inundation durations from BEWARE 2.0 and HyCReWW were 4 and 7 hours, respectively. Beach elevations associated with 5-year return periods were identified as suitable minimum nesting elevations to mitigate inundation risk.

BEWARE 2.0 and HyCReWW results for Ras Baridi lacked direct real-world validation, prompting a methodological validation approach. Six validated XBeach 1D NH models in fringing reef environments globally were utilized for comparison against BEWARE 2.0 and HyCReWW. The comparison revealed increased median scatter indices and root-mean-square (RMS) errors for both BEWARE 2.0 (0.07 and 0.1 m increases) and HyCReWW (0.1 and 0.15 m increases) with low-resolution bathymetry (10 m horizontal). BEWARE 2.0 had lower median RMS errors and scatter indices for high-resolution (2 m horizontal) and low-resolution bathymetry than HyCReWW, as well as smaller spreads. Consequently, utilizing BEWARE 2.0 results is recommended for Ras Baridi and similar data-scarce coastal environments over HyCReWW.

In summary, this thesis addresses the pressing issue of sea turtle nest flooding, providing a methodology to assist in responsible nest relocation in low data environments. The 5-year return period runup elevation of the BEWARE 2.0 results (1.25-1.75 m along the beach) has been identified as a suitable minimum nesting elevation in Ras Baridi, and the likelihood of flooding as a function of beach elevation has been provided. These results can aid coastal managers in making informed decisions for the protection of these endangered species.

Many sand spits are morphodynamically complex cases that are hard to quantify. Therefore, most case studies on sand spits in a specific type of environment, are descriptive or even non-existent. As is the case for spits in low-energy lake environments, like the two spits at the Marker Wadden islands. Because quantification is necessary for spits in such complex environments, sedimentation was quantified around both spits using polar coordinates and morphologically simplified elevation levels around the spit-platform. It was found that, sediment rich currents that pass over the spit-platform boundary at a certain direction drive spit-platform growth in that direction. The spit-platform growth in turn dictates the growth of the emerged spit. Spit growth quantification enables a detailed understanding of the Marker Wadden spits, which gives insights in spit behaviour in low-energy lake environments. The quantification of spits in other environments could be a powerful tool for the understanding of spit behaviour.

Coastal protection is required to keep coastal areas protected during storm conditions. Coastal dunes are a natural defense against flooding and erosion from the sea against storms. Many insights have been obtained regarding the positive effects of coastal dune vegetation on dune formation and growth in the past. Several recent studies have demonstrated the ability of both aboveground and belowground dune vegetation to reduce dune erosion during storms.

However, the impacts of dune vegetation during collision regime storm conditions are not taken into consideration in morphological numerical modeling. Consequently, it is unclear how to evaluate vegetation impacts during design assessments. Therefore, the goal of this research is to investigate the potential effect of dune vegetation on dune erosion during collision regime storm conditions and subsequently link this with dune rehabilitation projects. This research is divided into two parts.

First, the capability of the numerical model XBeach to simulate the potential effects of dune vegetation during the collision regime is investigated. Four vegetation approaches are identified in the model, which could possibly represent different dune vegetation effects. Using beach-dune profiles and erosion volumes obtained from two wave flume experiments, the performance and sensitivity of the vegetation approaches are tested in XBeach. Thereby, a distinction is made between the effect of aboveground vegetation: hydrodynamic altering and belowground vegetation: soil stabilization. The model results show that XBeach is capable of simulating dune erosion with vegetation during collision regime storm conditions. This can be primarily attributed to the increase of the critical slope in the avalanching module. This approach represents the soil stabilization effects of belowground vegetation. The application of the root model was added for even better simulations. This approach accounts for the additional root cohesion provided by belowground vegetation by increasing the critical velocity for sediment pickup. The values to be used for both vegetation approaches could not be defined systematically yet due to the small number of experiments assessed and little research found in the literature. It is recommended to obtain more information about the effect of dune vegetation on the stabilizing effects and the critical slope for avalanching. The application of a higher roughness value and the vegetation module, which both represent aboveground vegetation by altering the hydrodynamics, have shown to contribute insignificantly to erosion reduction in the examined cases.

The belowground soil stabilization approaches are applied in a case study to give a first indication of the applicability of XBeach at large-scale and in dune rehabilitation projects. An XBeach model of Beira, Mozambique was set up. A higher critical slope demonstrated profile evolution and erosion reduction in line with observations in the literature. During wave impact, the avalanching module is the most suitable approach to account for the erosion-reducing effect of belowground vegetation. The root model appears to be a more appropriate strategy for accounting for belowground vegetation during milder conditions and shorter storm duration. It was proven that XBeach has very good potential to evaluate the effectiveness of dune vegetation in the design phase. The case study illustrates that vegetation will significantly increase the erosion resistance of the flood dunes. The dunes with vegetation can handle an additional 20-30 centimeters design water level or 0.5-0.8 meter storm wave height without breaching compared to the same dune without vegetation. The design with vegetation has shown to be more robust. However, the approaches are based on different assumptions and limitations and therefore the results should be considered carefully. It is strongly advised to conduct more research in this relatively new study area.

This study investigates and proposes a method for quantifying the advantages of vegetation on dune erosion reduction. The relevance and added value of mature and robust dune vegetation for a resilient coastal dune system and protection against erosion and floods are confirmed. As a result, the incorporation of vegetation in dune rehabilitation projects is promoted.

During the calibration of SWAN for shallow water lakes in the Netherlands, problems have been encountered. Especially the emergence of an unnatural, secondary peak in the low frequency part of the energy density spectrum poses problems. Therefore, this investigation aims to address the nonlinear shallow water source terms in SWAN that may introduce this artefact.

Firstly, the basics of linear wave theory and spectral modelling are introduced. Hereafter, these basics are extended to the modelling on nonlinear wave behaviour, as these phenomena form the basis of the relevant shallow water source terms.

The shallow water lake problem is reduced to a stationary, one dimensional problem, for which several model results are produced using varying parameters. Three main subjects can be identified: 1) the influence of depth-induced wave breaking using the BJ78-model or the $\beta-kd$-model, 2) the proportionality factor of the triad source term using the CCA-formulation, and 3) numerical properties such as frequency resolution and the magnitude of the flux limiter.

The results demonstrate that, for strong winds ($U_{10} = 40$ m/s) in shallow water ($d = 2.5$ m), source term interactions become complex as they are all of similar order of magnitude. Especially the interactions between triad wave-wave interactions and depth-induced wave breaking, and triads and quadruplets are important for determining the resulting wave spectrum. Increased wave breaking reduces the secondary peak but underestimates significant wave heights and dampens the spectrum. A more elaborate wave breaking formulation (the $\beta-kd$-model) showed improved performance, especially when it was set to allow younger wind waves (high $kd$) to develop.

Lowering the effect of the triad source term also removes the secondary peak, but may not be desirable as it is by definition a relevant shallow water source term. Reducing source term interactions by means of a stricter action limiter resulted in instabilities for the quadruplet source term and thus no convergence of the results.

A possible solution may be to investigate the possibility of reducing the effect of quadruplets for shallow water, as this term (together with the triad source term) contributes to the formation of the secondary peak. However, this has not been included in the scope of this investigation. Furthermore, improving the frequency resolution resulted in smoother spectra and less wiggles in the sum of the triad and quadruplet source terms. However, this may affect the performance of the quadruplet source term as this term has been calibrated for a fixed relative frequency resolution.

Infragravity (IG) waves (0.005-0.04 Hz) are surface waves that can dominate the nearshore hydrodynamics and can impact various coastal processes (e.g., run-up, overwash). A proper offshore description of incident IG waves is required for storm impact models, which generally assume a local equilibrium between sea-swell (SS) waves (0.04-0.33 Hz) and the resultant nonlinearly excited bound IG waves. The contribution of free incident IG waves and the directional properties
of the IG wave field are neglected, though they play a critical role in IG wave variations. Various research focuses on IG wave dynamics in shallow waters, but a detailed understanding of IG wave variability in intermediate water depth is lacking. Furthermore, the directional spectra of bound and free IG waves in the field are usually unavailable since bound IG waves do not follow linear dispersion relations, which is assumed by commonly applied directional spectra reconstruction methods.

In-situ observation data (surface elevation, pressure, and velocity) of IG waves from November 2021 to April 2022 at three cross-shore locations in intermediate water depth (∼ 6 − 14 m) off the Dutch coast are analyzed to study generation, propagation, and directional properties of IG waves. The bispectral technique is applied to quantify the contribution of bound IG waves to the total IG wave field. A newly developed method is validated and used on the offshore (∼ 14 m) data to reconstruct directional spectra of bound and free IG waves separately during storms.

The results show that IG wave heights are best predicted (correlation coefficient R² up to 0.94) with an offshore forcing parameter that includes peak period H_ssT_p² , which indicates SS wave energy flux. The growth rate of IG waves is in between the shallow water equilibrium solution (for bound waves) and Green’s Law (for free waves), indicating the total IG wave field consists of bound and free components. The relative contribution of bound IG energy (up to 76%) is correlated to SS wave energy, whereas it decreases dramatically during intense storms when SS waves break. The bound IG waves that are assumed to follow the weakly nonlinear wave theory, have a similar peak direction and broader directional spreading to SS waves. In contrast, the directional spectra of free IG waves are nearly isotropic during calm conditions, but may have diverse peaks during storms, which incident IG waves from remote sources and edge waves can
influence. A better understanding of the complex pattern of free IG waves requires more detailed observations and/ or modeling.

Sea level rise causes more difficulties for coastal maintenance. Although hard structures were built in the past to prevent for flooding, nature based solutions are preferred more recently. For the application of nature based solutions, a better understanding of the coastal processes is desired. Dry sand is transported towards the dunes by aeolian processes, whose bedform development is understudied while knowledge on this is important. One of the most visually clear bedforms are organised dry sand over a moist beach, referred to as sand strips. In order to improve the knowledge on sand strips, this study focusses on sand strip properties and occurring environmental conditions during their presence at the Noordwijk beach, using data of a terrestrial laser scanning (TLS) device.

Sand strips are detected with the Fourier transform. Since surface moisture can be derived from the reflectance intensity of the TLS-data, and due to the different moisture content of the sand strips compared to the surrounding beach, the Fourier transform is applied on the reflectance intensity. Sand strips are detected based on the energy in the variance density spectrum for a wavenumber-range corresponding to sand strips. The detected sand strips were oriented alongshore to oblique-alongshore with a mean wavelength and height of 13.2 m and 4.0 cm respectively, which is in correspondence with similar sand strip-related studies.

According to sand samples of the beach the grain size varies in transverse direction of the sand strips, comparable with the grain size variation of aeolian sand ripples. The coarser grains were located at the crest and the finer grains at the lee-side. Additionally, the samples also showed a significant difference in gravimetric moisture content between the sand strips and surrounding beach, as expected due to the reflectance-based detection. At the sand strips, the maximum moisture content was 5.3%, while the minimum determined moisture content at the surrounding beach was 6.0%. The mean values were equal to 2.6% and 9.4% for the sand strips and surrounding beach respectively.

In addition, sand strips mainly occurred during (almost) alongshore wind events with a wind velocity in excess of 8 m/s. However, the threshold wind velocity for sand strip formation is determined at 10 m/s. Due to the significant height difference that can remain present during precipitation events, these events are not necessarily restrictive factors for sand strip development, although the reflectance intensity suggests different. Furthermore, sand strips mostly formed during falling tide and they were mostly destroyed during rising tide.

The one life cycle of the sand strips that is analysed showed dynamic sand strip behaviour. The migration rate of the sand strips varied over the width of the beach, causing more inland oriented sand strips. This dynamic behaviour cannot be related to weather conditions since they remained constant, however it could be related to topographic steering caused by the dune. Nevertheless, the results of the dynamic properties are only indicative and encourage further study of dynamic sand strip properties.

Sea turtles are a popular tourist attraction that offers travelers a unique nature experience. As a part of Saudi Arabia’s 2030 vision, an unprecedented amount of tourism development is being carried out under the ‘Red Sea Development Project’. The aim is to develop hyper-luxury islands in a sustainable and ecologically friendly way by closely following ‘Building with Nature’ principles. The tourism development site at Al Wajh Bank encompasses an archipelago of more than 90 islands and some of the sandy islands provide nesting grounds for Hawksbill and Green turtles. The thesis is a component of a broad study on investigating the preferential conditions for sea turtle nesting and conservation where TU Delft is collaborating with NIOZ and KAUST. The present study makes a preliminary assessment of the preferential conditions for sea turtle nesting of wave hydrodynamics and geomorphological conditions based on available data and numerical wave model simulations. The study is expected to provide information and guidance for future research for the conservation of sea turtle nesting at Al Wajh Bank.

The coral reefs provide breeding grounds for sea turtles in the gently sloping sandy beaches along the raised islands where the sand has ideal conditions of moisture, temperature, and distribution of sediment size for successful nesting and hatching of eggs at an optimum distance from the highwater line to avoid inundation by wave runup. In general, the foreshore at the turtle nesting site has a fore reef with a steep slope followed by a wide and shallow reef flat having high bottom roughness for dissipation of wave energy. The Al Wajh Bank spread over an area of 2880 km^2 has a very large and complex barrier reef system at the outer edge in the deep sea enclosing a massive lagoon with several islands. These islands have sandy beaches on the up wave (windward) side and mudflats with sand and vegetation in the sheltered zones. The vegetation is dominated by mangroves in the mudflats and seagrass in the shallow lagoon beds.

The study used the Delft3D WAVE stand-alone phase averaging spectral wave model for transforming waves from offshore to nearshore and to examine the energy dissipation characteristics of the reef system. The high-resolution bathymetry data for the Al Wajh Banks was obtained from GCC and relatively coarse resolution data for the offshore from GEBCO. The model simulated offshore wind and wave data validated using satellite altimetry, SAR, and Buoy measurements was obtained from BMT ARGOSS online services website “waveclimate.com”. The normal and extreme wave conditions were derived using this data. The data on GPS coordinates at the location of turtle nesting sites was provided by a survey undertaken by KAUST. A limited data on sediment size distribution at four islands south of Al Wajh Bank was made available by KAUST collected by ALS Arabia. The data on beach slopes, the distance of nests from HWL, geomorphology, vegetation was derived from secondary sources or in an indirect manner using bathymetry data.

The wave model results were analyzed for the nearshore wave heights and distribution of energy density at the fore reef and inside the lagoon. The wave model results revealed that the extensive barrier reef on the seaward side of the Al Wajh Bank is able to completely prevent and dissipate the energy of the offshore waves providing ideal conditions for turtle nesting at seaside islands. Further, the results show that the offshore waves do not have any role in the production and dissipation of wave energy inside the lagoon. The study confirmed that the waves inside the lagoon are exclusively local wind-generated waves. The wave climate inside the lagoon during storm conditions is only influenced by the prevailing strong winds and not dependent on the storm-generated extreme waves from offshore.

Most of the sea turtle nests have been found on the up wave or windward side of beaches with flat and wide reefs or fringing reefs between the reef crest and the high-water line. The wave model demonstrated the large energy dissipation rates under these geo-morphological conditions. In the absence of relevant data on sediments for the nesting and non-nesting beaches, no specific conclusions could be drawn. The beach slopes where sea turtle nests were abundant had slopes in the range of 1:10 and 1:20. The study indicated that the non-nesting sites inside the lagoon are in the sheltered zones of inner reef shelfs and behind the islands where mudflats with mangroves and other vegetation are abundant.

The wave runup is estimated using the HyCReWW metamodel at the nesting and non-nesting sites. The metamodel estimates indicated that the wave runup is of comparable magnitude both for the nesting and non-nesting sites. The comparison of runup distance along the beach indicated that the turtle nesting sites are located sufficiently away from the runup computed for 1 in 100-year wave height

A satellite imagery-based global shoreline data source (Shoreline Monitor, Deltares/TUDelft) was used to examine the erosion and accretion trends at Al Wajh Bank. The data identified two of the islands as having chronic erosion which have a small percentage of nesting sites. The analysis of the data identified that the turtle nesting sites are located on beaches with stable erosion/accretion rates.

The study confirmed the limitations of the Delft3D WAVE (SWAN) model used in simulating IG waves. Due to the very large domain and the course grid adopted in the present study the wave setup aspects have not been covered. The present study was useful in providing a range of information and guidance for further hydrodynamic and geomorphological data inputs at a regional scale and application of models like X-Beach or SWASH for a more comprehensive and detailed analysis on the influence of the surf and swash zone dynamics including IG waves on the sea turtle nesting sites. A systematic data acquisition on geomorphological and sediment characteristics will be useful for logistic regression analysis to study the dominant factors influencing the turtle nesting and non-nesting sites.

Each year several fatalities due to rip currents are reported in the Netherlands. One of these accidents was during May 11th in 2020 near the Scheveningen harbour, were several surfers lost their lives. During the hydrodynamic and meteorological accident were quite severe, with a significant wave height of 3.0m and wind speeds of 11 m/s, which is likely to have resulted in a boundary rip at the northern mole. On top of that extraordinary amounts of sea foam amassed at the northern harbour mole, disorienting anyone who got trapped in the area. It is likely that due to the wind and wave conditions and the resulting rip current it was near impossible for the surfers to exit the vicinity of the mole, especially with thick layer of foam. The Scheveningen harbour area was modelled in Delft3D and subjected to tide wind and waves. Assessment of the influence of wind, waves and tide on rip current intensity was performed by comparing the model results from a combined run with the rip velocities obtained by superimposing rip velocities per individual forcing condition. Lastly, the model was run with conditions resembling the aforementioned accident as to check whether the model was able hindcast this situation. The main wind and wave directions found to lead to offshore directed rip transport come from 30-70 degrees relative to shore normal. Retentive rips also occur, mainly when the wind-wave forcing comes from -70 to 0 degrees relative to shore normal, although rips occur less frequently in this range.